1. Field of the Invention
The present invention pertains to a method of fabricating a hinge, and more specifically, to a method of performing a wafer level test in situ and adjusting the characteristic of a hinge according to the wafer level test.
2. Description of the Prior Art
Micro-electromechanical system (MEMS) technology is an emerging technology which highly integrates electronics and mechanics, and has been broadly adopted to fabricate various devices having electro and mechanical characteristics, such as micro sensors, micro actuators, micro motors, photosensitive switches, etc. MEMS devices generally have mechanical structures more complicated than semiconductor devices, and thus cannot be directly fabricated by standard semiconductor processes. Take the micro hinge, one of the most common structures in MEMS devices, for example, accurate shape and smooth surface conditions are strictly required for ensuring reliability and stress-bearing capability.
Please refer to FIG. 1 to FIG. 3. FIG. 1 and FIG. 2 are schematic diagrams illustrating a prior art method of forming a micro hinge 10, and FIG. 3 is a schematic diagram of a single-axis micro hinge 10 made by the prior art method. As shown in FIG. 1, a wafer 20 is first provided. An etching stop layer 22 and a photo resist pattern 24 are respectively formed on the bottom surface and the top surface of the wafer 20. As shown in FIG. 2, the bottom surface of the wafer 20 is fixed on a carrier 32 by a bonding layer 28, and an etching process is followed. Portions of the wafer 20, which are not covered by the photo resist pattern 24, are etched through until the etching stop layer 22 is reached.
As shown in FIG. 3, the micro hinge 10 is a suspension structure capable of being driven by a voltage, light beams, or a magnetic field, and thereby rotates in the direction as the arrow indicates shown in FIG. 3. Therefore, the micro hinge 10 must have an accurate shape, a smooth surface, and a uniform axis so as to guarantee reliability and stress-bearing capability.
According to the prior art method, however, equilibrium of the etching process and the thickness uniformity of the wafer 20 are not taken into consideration. Thus, the yield of the etching process cannot be controlled well because etching rates in different regions of the wafer 20 are not equal. For example, on the occasion of etching through the wafer 20, the overall area being etched varies dramatically, and leads to unexpected changes during the etching process. In addition, when the etching process is performed down to the etching stop layer 22, side-etching effect tends to occur and therefore results in an undercut 26 as shown in FIG. 2. As mentioned earlier, if the shape accuracy of the micro hinge degrades, reliability of the micro hinge will be seriously affected.
On other hand, according to the prior art method of forming the micro hinge 10, the thickness of the wafer 20 is exactly the thickness of the micro hinge 10. This leads to a great limitation in changing the thickness of the micro hinge 10. Furthermore, because the bottom surface of the micro hinge 10 is fixed on the etching stop layer 22, the micro hinge 10 cannot accept a wafer level test in situ, and be adjusted immediately. The step of removing the etching stop layer 22 might even damage the micro hinge 10.